In high-pressure valves of this type, as used for instance in nuclear reactors, the valve element is generally carried at one end of a valve stem which is slidably received in a housing mounted on the body of the valve, sealing means being provided to seal the valve stem with respect to this housing. In a conventional embodiment of such a high-pressure valve the valve stem is secured to or forms part of a threaded operating spindle projecting from the housing and carrying a handgrip for the manual operation of the valve. As a consequence of the small width of the valve passage, the valve element and the valve stem and operating spindle have a correspondingly small diameter and it will be clear that it is not possible to mount a heavy hand wheel or similar handgrip on such a small sized spindle. In many cases a bellows seal or diaphragm seal is employed as sealing means for the valve stem. However, for structural reasons the diameter of such a bellows or diaphragm seal can not be smaller than approximately 25 mm which means that this seal has a substantially larger diameter than the valve element. As a consequence,the axial force exerted by the pressure of the fluid to be controlled in the opening direction of the valve is in fact determined by the cross-sectional area of the bellows or the diaphragm. As a result thereof, the operating force to be applied for closing the valve becomes very great whereby not only twisting or bending of the relatively light sized spindle can occur but also the operation by means of the relatively small handgrip becomes heavy and difficult. Furthermore, the manually operated high-pressure valves with bellows or diaphragm seals generally have a very small stroke of the operating spindle between the closed and the open position. For this reason it is difficult to provide a clear signalling means to indicate the position to which the valve has been moved.